Continuously variable transmissions (CVT) of the variable pulley or sheave type employ pulley assemblies having at least one member that is moveable to control the diameter at which a flexible transmitter, such as a belt or chain, operates. The transmission has an input pulley and an output pulley, both of which have an adjustable member. The transmission ratio between the input and the output pulleys varies between an underdrive ratio and an overdrive ratio.
The CVT ratio is continuously variable between the extremes of the underdrive and overdrive ratios. During the underdrive ratios, the flexible transmitter is positioned at a small diameter on the input pulley and a large diameter on the output pulley. Thus, the input pulley has more than one revolution for each revolution of the output pulley. As the diameter of the input pulley increases, the diameter of the output pulley decreases until a 1:1 ratio exists across the pulleys. During the overdrive ratios, the diameter of the input pulley is maintained larger than the diameter of the output pulley. Thus, each revolution of the input pulley results in more than one revolution of the output pulley.
To accommodate the ratio variance, at least one member of each of pulley is disposed to slide axially relative to the other member of the pulley. This movement is typically controlled hydraulically.
CVTs have become increasingly popular in recent years because they may provide improved fuel economy, the ability to operate the engine at lower rpms over a wider range of the fuel economy schedule, smoother shifting (ratioing), more efficient vehicle front end packaging, as well as manual transmission interchangeability and all-wheel drive compatibility.
In current CVT designs, the rotatable pulley members are supported on bearings, and the bearings rotate within steel sleeves which are pressed into a bore in the case or cover. These steel sleeves are required in order to reduce the wear between the bearings and the aluminum case or cover. Accordingly, the steel sleeves add a component cost and additional assembly costs because the sleeves must be pressed into the bores. Further, the steel sleeves have a wall thickness of about 1.5 mm. As a result of packaging constraints, the CVT is designed to be as small as possible so that the 1.5 mm thickness of the steel sleeves limits the bearing outside diameter by 3 mm.